The term oil shales covers a wide variety of laminated sedimentary rocks containing two main components:
(1) Organic matter, that can be released by destructive distillation, and PA1 (2) mineral constituents which consist mainly of clay, calcite, dolomite and iron compounds. PA1 valuable fuel products from the organic matter, present in the low-grade oil shales, and PA1 a catalytic effect of the mineral moiety of oil shales.
Oil shales deposits occur in many countries of the world and in sedimentary rocks of virtually all ages. These deposits vary widely as to size, constitution and strength of the shale. Broadly speaking two main categories of oil shales can be distinguished: oil shales which are rich in organic matter and contain relatively low mineral constituents, and those which are low in organic matter and rich in mineral constituents. Thus for instance, oil shales from Green River (U.S.A.) contain 22.1% CaO and a 70% conversion of organic matter to oil, whereas these from Israel contain 64.8% CaO and said conversion only 48%. On the other hand, oil shales from Iraty (Brazil) contain only 2.6% CaO and a very high conversion of the organic matter to oil. These figures clearly indicate the big differences which exist in the constituents of oil shales from various sources and their corresponding use.
Application of heat is the only means that has been found to produce oil from these shales and numerous mechanical devices known as retorts, have been developed for this purpose. Retorting involves the crushing and heating oil shales at high temperatures of about 500.degree. C., followed by cooling and discharging of almost equally large quantities of spent shales. While this method is feasile and even applicable for oil shales which are relatively low in organic matter such as in Autumn (France), it was not considered economically to be utilized for oil shales which are low in organic matter but contain high amounts of minerals, such as in Israel.
The first recorded reference on processing oil shales is a U.K. Patent dated 1694, in which oil was distilled out from "the stone". Later on, many processes were developed based on retorting, inserting improvements for the recovery of as much as possible of the energy present in the oil shales. The main disadvantages of retorting are high-energy requirements, low conversion of the organic matter to liquid and the presence of olefins and heteroatoms in the product. Attempts were tried to obviate the energy requirements, by utilizing solvent extraction of the oil shales at ambient temperature. Using common solvents for petroleum, such as benzene, acetone, carbon disulfide, etc., only a very small amount of the organic matter could be extracted.
As known, the organic matter in oil shales is mainly an insoluble,solid material called kerogen, which has a high molecular weight being of a polymeric nature. Oil per-se is not present in oil shales, but upon its pyrolysis at about 500.degree. C., a liquid hydrocarbon with some gaseous products and a solid residue is generated. There are some prior patents claiming the extraction of oil shales kerogen in common organic solvents at temperatures in the range of 200.degree. C. to 400.degree. C. Significant amounts of oils were indeed extracted at these temperatures, reaching up to 85% at 350.degree. C. but excessive vaporization and cracking of the solvent rendered the process to be non applicable. In the U.S. Pat. No. 3,972,724 a process is described for processing fuel shales to produce energy and cement clinker. According to the process, the fuel shales are admixed with finely divided SiO.sub.2, CaO and R.sub.2 O.sub.3 and combusted in a steam boiler plant at a combustion temperature up to about 1400.degree. C., generating heat and a residue from which a cement powder is produced.
The conclusion which can be reached from the prior art on this subject is that the solvent extraction based on a supercritical fluid, seems to have some advantages which appear to outweigh their disadvantages and should be considered as the most promising route for the exploitation of oil shales.
There are some references which deal particularly with the processing of oil shales which contain high amounts of mineral constituents. In a report by A. B. Vol Epshtein et al (Khimiya Tvendogo Tapliva, Vol.14, 2, 67-70, 1980), there are summarized some experiments on the thermal dissolution of Baltic combustible shales, carried out in a flow-through apparatus. The oil shales were heated for 6 to 10 hours at a temperature in the range of 400.degree. C.-430.degree. C. at a pressure of 30 to 50 atmospheres, in the presence of a solvent which has a boiling point in the range of 210.degree. C. to 340.degree. C. The ratio between the solvent to oil shales was 1.3 to 1. Under these conditions, due to a cracking reaction, occurs a degradation of the organic matter present in the oil shales which are transformed into a soluble state, obtaining a heavy oil-extract as the main product. This extract is mentioned to be useful as a power fuel, as a raw material for the production of electrode coke, as bitumen for road building and as a crude oil for obtaining hydrocarbons. The separation of the resulted liquid products is carried out by distillation. In the Israeli Patent No. 25377, there is claimed a process for the burning of low grade fuel,such as oil shales with limestone, in a suspended state at a temperature above 1000.degree. C. in a fluidized-bed furnace. It is mentioned that at least 30% of the thermal energy required in the process, is obtained from the raw materials.
The above brief review clearly indicates the interest which is attributed to the subject of exploitation of oil shales by many scientists and especially in these days, looking for an alternative source of fuel to be produced in a most economical way.
It is an object of the present invention to provide a process for the exploitation of low grade oil shales. It is another object of the present invention to provide a process in which the two main constituents of said low grade oil shales--organic matter and mineral moiety--are efficiently utilized. It is a further object of the present invention to provide a process for the utilization the ash resulted in the processing of any oil shales, producing useful products,